1. Field of the Invention
This invention relates to an electrical insulating oil composition, a capacitor impregnated with the electrical insulating oil composition, a method for producing the capacitor and a method of operating the capacitor. More particularly, the invention relates to an electrical insulating oil composition which has high hydrogen gas absorbing capacity and can be employed in a capacitor operating at a very wide temperature range, for example, at room temperature to temperatures as low as -40.degree. C. or -50.degree. C.
2. Description of the Prior Art
In the 1960's, polychlorinated biphenyl (PCB) was widely used as the insulating oil for high-tension capacitors for electric power supply. After the toxicity of PCB became an issue, various kinds of insulating oils have been proposed in place of PCB. The insulating oils which were industrially produced in 1970's as substitutes for PCB are classified into two groups. One group includes the mixture of chlorinated alkyldiphenyl ether, phthalic acid ester and benzene trichloride; and benzyl alcohol and esters of fatty acids; with which the oil having a high dielectric constant like PCB was aimed. The other group is exemplified by bicyclic aromatic hydrocarbons such as phenylxylylethane (PXE) and monoisopropylbiphenyl (MIPB). These insulating oils have an advantage in a partial discharge characteristic as compared with the former ones which have a high dielectric constant. Furthermore, the insulating oils of the latter group are low in viscosity, excellent in impregnating property, especially in the infiltration into spaces among layers of films, which enabled the industrial production of all-film-type capacitors (plastic film is used in place of insulating paper).
With the wide spread of all-film-type capacitors in 1980's, the production of insulating oils of the former group having a high dielectric constant was stopped because they are inferior in partial discharge characteristic and impregnating property, in addition, the advantage of the high dielectric constant hardly contributes to the performance of all-film-type capacitors.
With regard to the insulating oils of bicyclic aromatic hydrocarbons, several proposals have been made in order to improve their properties further. For instance, the ratio of aromatic portion (aromaticity) is increased in order to improve the partial discharge characteristic. More particularly, the molecular weight is lowered by reducing the number of aliphatic carbon atoms with maintaining the bicyclic aromatic structure. Such the insulating oil is exemplified by benzyltoluene disclosed in Japanese Patent Publication No. 55-5689. A good partial discharge characteristic can be expected of the benzyltoluene because the compound is low in molecular weight and high in aromaticity as compared with the foregoing MIPB and PXE.
With the use of the insulating oils of bicyclic aromatic hydrocarbons in place of PCB, the all-film type capacitors could be put on a commercial basis and the low temperature characteristic of the product could be improved. It is considered that the above advantages are brought about by the improvement in viscosity and pour point at lower temperatures which improve the partial discharge at lower temperatures.
As for the foregoing period in which PCB was used, according to the standard of IEC for insulating oils (Publication, 588-3 (1977), Askarels for Transformers and Capacitors), the viscosity and the pour point are prescribed as follows:
The Type C-1 for capacitors is a mixture of the isomers of dichlorobiphenyls and trichlorobiphenyls and it is prescribed that the viscosity is 30 to 40 cSt (.times.10.sup.-2 cm.sup.2 /sec) at 20.degree. C. and the pour point is -24.degree. C. With regard to trichlorobiphenyl of Type C-2, the viscosity is 41 to 75 cSt (.times.10.sup.-2 cm.sup.2 /sec) and the pour point is -18.degree. C., which pour point is relatively high. Accordingly, the behavior of the characteristics of capacitors in the lower temperature region near and below the pour point is a serious question in the designing of capacitors. As the method for investigating such a behavior of the low temperature characteristics, there is EDF Test Method that is proposed by Electricite de France and is employed on a world-wide level. In this test method, samples are cooled to -25.degree. C. in a refrigeration chamber during the night, and in the next morning, they are taken out of the refrigeration chamber and at an ordinary temperature, they are applied with electric voltages containing impulses which will occur in a transition phenomenon, thereby investigating their durability. The efficiency was confirmed by repeating such an operation every day for a long period of time. In other words, the temperature of -25.degree. C. was considered as a critical temperature for this period as will be understood from the foregoing description on the viscosity and pour point. When devices are started at temperatures lower than this temperature, it was considered to be a good method for starting to warm up by, for instance, gradually applying electrical loads.
As the solid insulating substance to be used together with PCB, insulating paper or combined films of insulating paper and biaxially oriented polypropylene film (PP-film) was employed. However, the power loss as the whole capacitors were increased, especially at lower temperatures, because the power loss of both paper and PCB is large. For example, the loss at temperatures of -10.+-.20.degree. C. is approximately 0.1%, meanwhile the loss is abruptly increased by ten times to 0.1% at temperatures of -20.degree. C. to -30.degree. C. For this reason, the generation of heat in the capacitor becomes large and the temperature rise of 20.degree. C. to 30.degree. C. is caused to occur by heat generation, which depends upon the sizes of capacitors and the configurations of solid insulating materials and electrodes. As a result, even when the temperature of an insulating oil is at a pour point or below, the temperature is gradually raised by the internal heat generation of the capacitor, the temperature thus exceeds the pour point of the insulating oil in due course, and finally, the viscosity is lowered and the insulating oil can act as a liquid substantially. Accordingly, in the above-mentioned EDF Test, in the process of the change of an insulating oil from a solid state to a liquid state during the electrical loading, even when partial discharge is caused to occur in the initial stage, it is ceased with the passage of loading time. As described above, the change of power loss, the accompanying temperature change, the change of the state of insulating oil, and the condition of partial discharge are entangled in said method, thereby determining the final deterioration in the characteristics of a capacitor and its overall durability such as dielectric breakdown. This test method excels in that various factors and their interrelation can be evaluated collectively. The thus obtained results are, however, too complicated to analyze the test determinative factors. This test was developed mainly to test the appliances impregnated with PCB. Therefore, the drawbacks of this test method are no more than the undesirable behavior that is brought about by the characteristic properties of PCB as an insulating oil. A new test method for newly developed insulating oils is necessary from the above viewpoint.
Meanwhile, the bicyclic aromatic hydrocarbons such as PXE and MIPB that have come on as substitutes for PCB are now used for the all-film type capacitors as leading insulating oils. The pour points of them are below -50 C, with which the low temperature characteristics were surely improved.
However, the viscosity near the pour point is very high. For example, the viscosities of MIPB and PXE at -50.degree. C. are above 10,000 cSt (.times.10.sup.-2 cm /sec). The high viscosity like this is not desirable because the diffusion of the hydrogen gas that is released in partial discharge is hindered. Therefore, desirable viscosity is below 2000 cSt (x 10.sup.-2 cm.sup.2 /sec), and more preferably below 1000 cSt (.times.10.sup.-2 cm.sup.2 /sec).
Though the dielectric loss of these bicyclic aromatic hydrocarbons varies according to the shapes of electrodes and impurities in insulating oils, it is on the level of about 0.01% to 0.02% which is one tenth of the capacitor with PCB. Even at temperatures as low as -40.degree. C., the dielectric loss does not exceed 0 1%. Accordingly, it is a characteristic feature that the temperature rise in a capacitor owing to the dielectric loss is less than 5 C. In other words, the dielectric loss increases with the lowering of temperature in the case of PCB, however, in the case of the bicyclic aromatic hydrocarbons, the compensation by the heat generation of dielectric loss cannot be expected in low temperature conditions, especially in extremely low temperature conditions of -40.degree. C. to -50.degree. C. Accordingly, it is inevitable that the insulating oil itself can fully withstand the low temperatures, that is, in liquid at a very low temperature.
The insulating oils of bicyclic aromatic hydrocarbons that are used at present are the foregoing PXE and MIPB; and the mixture of monobenzyltoluene (MBT) and dibenzyltoluene (DBT). Any of these substances has a low temperature characteristic that is superior to that of PCB. In order to improve further the adaptability and the partial discharge characteristic at lower temperatures, the inventors of the present application have made detailed investigation regard to the relation between the structures of non-condensed bicyclic aromatic hydrocarbons and the properties of them as electrical insulating oils.
In the first place, alkyl groups having 1 to 5 carbon atoms were added to the skeletal carbon chains of 1,1-diphenylethane so as to synthesize the model compounds of the basic skeletal structure of bicyclic aromatic hydrocarbons. The properties as synthetic oils were investigated with regard to the six kinds of synthetic oils including the compound having only the basic skeletal structure.
The structures of the synthetic oils are represented by the following structural formula: ##STR1## wherein R is a mixture of a methyl group, two methyl groups, an ethyl group, an isopropyl group, a tert-butyl group or a tert-amyl group.
Each of the synthetic oils was refined by clay treatment to make the dielectric loss tangent below 0.02% at 80.degree. C., which was followed by several kinds of tests as insulating oils for capacitors. In order to observe the basic property as insulating oils, hydrogen gas absorbing capacity was measured, the results of which are shown in FIG. 1 According to these results, the hydrogen gas absorbing capacity increases with the decrease of the number of carbon atoms in substituent groups, i.e., with the rise of aromaticity (the percentage of aromatic carbons in the total structure). Taking the above fact into consideration, all-film type model capacitors were made by using the respective synthetic oils and their performance was tested as follows.
Two sheets of 14 micrometer thick biaxially oriented polypropylene films were put together to overlap each other. With using the thus prepared films as insulating materials, aluminum foil of 7 micrometers thick was wound to obtain capacitors of 0.3 to 0.4 .mu.F.
Breakdown voltages were measured by applying electric voltage to these capacitors in a room at a temperature of 25.+-.3.degree. C. An electric voltage (2400 V) which corresponds to 50 V/.mu.m in potential gradient was applied to the capacitors for 24 hours and after that, the electric voltage Was raised by 10 V/.mu. at an interval of 48 hours. The number of capacitors was 6 for each synthetic oil and the times at which capacitors were broken down were recorded and their average was taken as the value of each group of capacitors.
The results obtained in the above tests are shown in FIG. 2. According to these results, the voltage withstanding characteristics become higher with the rise of aromaticities of the compounds, that is, the lowering of molecular weights, which correspond to the tendency of hydrogen gas absorbing capacities of the compounds shown in FIG. 1.
It was understood from the results shown in FIG. 1 and FIG. 2 that the hydrogen gas absorbing capacity and the voltage withstanding characteristic become better with the lowering of the molecular weights of bicyclic aromatic hydrocarbons.
The hydrogen absorbing capacity and voltage withstanding characteristic were measured with regard to the alkyl nuclear derivatives of diphenylmethane, those of biphenyl and also those of diphenylethane. As a result, the tendency of them in connection with the alkyl substituent groups relative to the total number of carbon atoms were similar to those shown in FIG. 1 and FIG. 2.
The viscosity becomes low with the lowering of molecular weight of bicyclic aromatic hydrocarbon, however, the melting point becomes high because the compound structure is simplified, which fact makes worse the low temperature characteristics.
In the following Table 1, the melting point of bicyclic aromatic hydrocarbon (non-condensed type) having 12 carbon atoms which is biphenyl and has a lowest molecular weight in the non-condensed type bicyclic aromatic hydrocarbons, and those of non-condensed type bicyclic aromatic hydrocarbons having 13 carbon atoms (the number of carbon atoms is larger by 1 than biphenyl) are shown.
The melting points of all of them are high, in addition, the flash points of them are low. Accordingly, they are not suitable as inevitable components for use in preparing electrical insulating oils or electrical insulating oil compositions.
TABLE 1 ______________________________________ Melting Points of Bicyclic Aromatic Hydrocarbons (Non-Condensed Type) Number of Melting Point Substance Carbon Atoms (.degree.C.) ______________________________________ Biphenyl 12 +69.1 2-Methylbiphenyl 13 -0.2 3-Methylbiphenyl 13 +6 4-Methylbiphenyl 13 +51.5 Diphenylmethane 13 +26.5 ______________________________________
According to FIGS. 1 and 2, in view of the hydrogen gas absorbing capacity and the breakdown voltage, the bicyclic aromatic hydrocarbon having 14 carbon atoms are most preferable among those having not less than 14 carbon atoms. Accordingly, it is considered that an electrical insulating oil composition having good low temperature characteristics at -40.degree. C. to -50.degree. C., can be prepared by using such the materials.
The bicyclic aromatic hydrocarbons having 14 carbon atoms are exemplified by dimethylbiphenyls, ethylbiphenyls, methyldiphenylmethanes, 1,1-diphenylethane and 1,2-diphenylethane; corresponding compounds having an ethylenic double bond such as vinylbiphenyls, 1,1-diphenylethylene and 1,2-diphenylethylene: and the position isomers and stereo-isomers of them.
The number of bicyclic aromatic hydrocarbons having 14 carbon atoms is particularly large as compared with those having 12 or 13 carbon atoms. It is quite impossible by the conventional method of trial and error to select suitable compounds from the former ones that are satisfactory in view of their properties and their industrial applications and to clarify the compositions and characteristics of insulating oils. In practice, any electrical insulating oil or electrical insulating oil composition of the bicyclic aromatic hydrocarbons having 14 carbon atoms which has advantageous properties at temperatures of below -40.degree. C. or more preferably -50.degree. C., has never been used.
In order to create a new electrical insulating oil composition which has excellent low temperature characteristics, the following study was made. In view of the properties and industrial utility, some promising compounds which are considered to be inevitable components for an electrical insulating oil composition having good low temperature characteristics, were selected from the bicyclic aromatic hydrocarbons having 14 carbon atoms. The behavior at low temperatures of the multi-component systems of these compounds were clarified in a manner which has never been tried in the past.
More particularly, there are 12 kinds of position isomers of dimethylbiphenyls. A method to methylate biphenyl is known as an economical method for synthesizing dimethylbiphenyls. In this method, methyl groups are often oriented symmetrically due to the orientation of the substituent groups. As a result, a mixture of symmetrical dimethylbiphenyls is obtained and the inclusion of high-boiling components cannot be avoided. The symmetrical dimethylbiphenyls are, for example,
2,2'-dimethylbiphenyl (melting point: +20.degree. C.) PA0 3,3'-dimethylbiphenyl (melting point: +9.degree. C.), and PA0 4,4'-dimethylbiphenyl (melting point: +122.5.degree. C.). PA0 .DELTA.H.sub.i.sup.f is the heat of fusion (cal.mol.sup..div.1) of said component i as a pure substance, PA0 T.sub.i.sup.f is the melting point (K) of said component i as a pure substance, PA0 T is the temperature (K) of the system, PA0 r.sub.i is an activity coefficient, and PA0 (a) m-ethylbiphenyl, PA0 (b) p-ethylbiphenyl, PA0 (c) o-benzyltoluene, PA0 (d) m-benzyltoluene, PA0 (e) p-benzyltoluene, PA0 (f) 1,1-diphenylethane, and PA0 (g) 1,1-diphenylethylene
Accordingly, the dimethylbiphenyls cannot be the inevitable component for the industrial electrical insulating oil composition having good low temperature characteristics.
Among ethylbiphenyls, there are 3 kinds of position isomers, o-ethylbiphenyl, m-ethylbiphenyl and p-ethylbiphenyl. In the industrial synthesis of these ethylbiphenyls, they are produced by ethylation of biphenyl or transalkylation of ethylbenzene with biphenyl, in which most of the products are m-ethylbiphenyl and p-ethylbiphenyl, while o-ethylbiphenyl is hardly produced in this method.
Accordingly, among the ethylbiphenyls, those which can be inevitable components for the electrical insulating oil composition having practically good low temperature characteristics are m-isomer and p-isomer.
Methyldiphenylmethanes (benzyltoluenes) are industrially produced and are practically used as electrical insulating oils, so that they can be promising compounds for the electrical insulating oil composition having good low temperature characteristics.
The melting point of 1,1-diphenylethane is as low as -18.degree. C., so that it can be a promising compound.
The melting point of 1,2-diphenylethane is as high as +51.2.degree. C. and the heat of fusion is large, so that it cannot be a component of the insulating oil because the temperature of crystallizing out becomes high even when it is contained as one component of an electrical insulating oil.
As disclosed in United States Pat. Nos. 4,493,943; 4,506,107; and 4,618,914, the bicyclic aromatic hydrocarbons having ethylenic double bonds are interesting compounds as the component materials for electrical insulating oils. Among them, there are 3 groups that have 14 carbon atoms, vinylbiphenyls, 1,1-diphenylethylenes and 1,2-diphenylethylenes (trans- and cis-stilbene). Among them, the vinylbiphenyls are not desirable because they are liable to polymerize. The trans-stilbene is out Of the question because the melting point thereof is as high as +122.degree. C. Even though the cis-stilbene cannot be used singly, it can be used by being mixed with other components. However, stilbenes, on the whole, have a conjugated structure, so that the influence of them on living bodies is apprehended. While, 1,1-diphenylethylene passed a mutagen test (Ames test) according to the investigation of the present inventors and it is considered that the compound is safer than stilbenes.
Accordingly, 1,1-diphenylethylene is only one practically available compound among the bicyclic aromatic hydrocarbons having 14 carbon atoms and ethylenic double bonds.
The melting point of 1,1-diphenylethane itself is low enough and it can be used as one component of the insulating composition.
From the above discussion, the compounds (a) to (g) in the following Table 2 are nominated for promising materials of the electrical insulating oil composition.
TABLE 2 ______________________________________ Melting Points and Heats of Fusion of Bicyclic Aromatic Hydrocarbons Having 14 Carbon Atoms Melting Point Heat of Fusion Compound (.degree.C.) (cal/mol) ______________________________________ (a) 3-Ethylbiphenyl (m-isomer) -27.6 4000 (b) 4-Ethylbiphenyl (p-isomer) +35.5 2810* (c) o-Benzyltoluene +6.6 5000 (d) m-Benzyltoluene -27.8 4700 (e) p-Benzyltoluene +4.6 4900 (f) 1,1-Diphenylethane -18 4200 (g) 1,1-Diphenylethylene +8.6 3450* Reference Examples 1,2-Diphenylethane +51.2 5560* trans-Stilbene +126 6330* cis-Stilbene +2 3710* 2-Ethylbiphenyl (o-isomer) -6.1 3890 ______________________________________
In Table 2, all the melting points were quoted from published references and the heats of fusion marked with asterisks (*) were actually measured by using Specific Heat Measuring Device, HS-3000 made by Shinku Riko Co., Ltd.
In a multi-component system, liquids are soluble to one another and, when components are solid, they are not mixed together and do not form any solid solution, and the solid-liquid equilibrium of multi-component system is represented by the following general equation according to thermodynamic theory: ##EQU2## wherein X.sub.i is the equilibrium mole fraction of a component i in the liquid phase of the multi-component system,
R is the gas constant (cal.mol.sup.-1.K.sup.-1).
According to the experiment of the present inventors, there is no problem by assuming that the above activity coefficient r.sub.i equals 1 at least in the bicyclic aromatic hydrocarbons having 14 carbon atoms as shown in the foregoing Table 2, so that the above equation will be used hereinafter with r.sub.i =1.
With regard to an arbitrary electrical insulating oil composition of multi-components, the proportion of solid phase (crystalline phase) to the whole at, for example, -40.degree. C. or -50.degree. C., the starting point of crystallizing out, and the eutectic point can be calculated by the ordinary calculation method of solid-liquid equilibrium using the above equation.
Some of hydrocarbons in the foregoing Table 2 are already proposed as electrical insulating oils in published references. The characteristics of these substances will be calculated according to the above solid-liquid equilibrium equation.
For example, disclosed in Japanese Patent Publication No. 55-5689 is the use of an electrical insulating oil of o-benzyltoluene and p-benzyltoluene. The melting points of these hydrocarbons are +6.6.degree. C. and +4.6.degree. C., respectively. An electrical insulating oil having good low temperature characteristics cannot be made even from the mixture of these two components, without saying the case in which any of them is used singly. Up to now, any electrical insulating oil of these hydrocarbons is not practically used.
In U.S. Pat. No. 4,523,044; a composition comprising, for example, the composition of benzyltoluene and dibenzyltoluene prepared from benzyl chloride and toluene with a metal halide such as FeCl.sub.3 and its preparation method, are disclosed. This composition is used as an electrical insulating oil. According to this reference, the low temperature characteristic is improved by mixing the by-product dibenzyltoluene to lower the melting point because the melting point of benzyltoluene is approximately -20.degree. C.
The synthesis method of examples in these references were traced by the present inventors. The results was such that the reaction using this FeCl.sub.3 is o- and p-orientation and obtained composition of benzyltoluenes was 48.9 mole % of o-isomer, 6.8 mole % of m-isomer and 44.3 mole % of p-isomer. With this composition, o-isomer firstly begin to precipitate at approximately -15.degree. C. according to the foregoing equation, and at -20.degree. C., more than a half of them separates out as crystals. Therefore, it is certain that the melting point is near -20.degree. C., so that the low temperature characteristic of these benzyltoluenes is worse and it cannot be used practically. Even when the by-product of dibenzyltoluenes are added to the benzyltoluenes, the effect of depression of melting point of the composition is small for the amount of addition, because it depends upon the mole fraction of added substance while the molecular weight of dibenzyltoluene is high. More particularly, even though 20% by weight of the by-product dibenzyltoluene is added to the benzyltoluenes obtained by the method described in the above reference, the value in mole % is 14.3, which lowers the temperature of crystallizing out by only about 7.degree. C. However, the addition of high molecular weight dibenzyltoluene as much as 20% by weight causes the significant increase of viscosity at low temperatures. If more dibenzyltoluene is added for depressing the melting point, the advantage in the low viscosity of benzyltoluene is much impaired, so that it is not practical.
The lowest temperature of crystallizing out of the mixture of three kinds of benzyltoluenes exists at the eutectic point calculated from the above solid-liquid equilibrium equation, at which the composition is o-isomer: 17.4 mole %, m-isomer: 63.4 mole %, and p-isomer: 19.2 mole %. The eutectic point is -38.9.degree. C. Accordingly, without saying the product of the synthesis of benzyltoluene as disclosed in the foregoing reference, in any isomer mixture of the three kinds of benzyltoluenes at any compounding ratio, the mixture cannot exist in a liquid state at temperatures as low as -40.degree. C. to -50.degree. C.
In ethylbiphenyls, three kinds of position isomers exist likewise. That is, o-isomer, m-isomer, and p-isomer, and among them, the melting point of m-isomer is lowest. The eutectic point of these three kinds of isomers is -45.6.degree. C. according to calculation using the above solid-liquid equilibrium equation, at which the composition is o-isomer: 28.1 mole %, m-isomer: 52.4 mole %, and p-isomer: 19.5 mole %. Accordingly, also in the case of ethylbiphenyls, the mixture of only the three kinds position isomers cannot exist in liquid phase at -50.degree. C.
Of course, the synthesis method which produces mainly two-component system of position isomers can be employed, for example, in the synthesis of benzyltoluene or ethylbiphenyl.
For example, as disclosed in the foregoing U.S. Pat. No. 4,523,044 on benzyltoluene, benzylchloride and toluene are reacted using a halogenated metal to synthesize o- and p-oriented products. Or, biphenyl is ethylenated by Friedel-Crafts reaction by using a halogenated metal to synthesize ethylbiphenyls, wherein a composition of 66 mole % of m-isomer, 34 mole % of p-isomer, and less than 1 mole % of o-isomer is obtained. These methods can produce mixtures of position isomers of two-component system.
When the number of components in a position isomer mixture is reduced, however, even if the mixture contains much position isomer having a low melting point, it is still undesirable because the melting point of the mixture is naturally higher than the foregoing eutectic point of three-component system.
The 1,1-diphenylethylene is an excellent electrical insulating oil as described in the foregoing patent publication, however, the melting point of compound itself is high as shown in the foregoing Table 2, so that it cannot be used singly. Furthermore, there is a possibility that the melting point of an alkyl derivative is low. It is not desirable, however, because the proportion of olefin within one molecule and the aromaticity are lowered.
As described above, it can be expected that the bicyclic aromatic hydrocarbons (a) to (g) having 14 carbon atoms indicated in the foregoing Table 2, are used as excellent electrical insulating oils. However, any one of them cannot be a liquid at temperatures as low as -50.degree. C. when they are used singly. Furthermore, it is apparent that even when they are obtained in a form of a mixture of position isomers by an ordinary synthesis method and the depression of melting point is expected, any electrical insulating oil which can be practically used at low temperatures of -50.degree. C., cannot be obtained.
Thereupon, the inventors of the present application made detailed investigation with regard to the behaviors of oil-filled capacitors at temperatures as low as -40.degree. C. to -50.degree. C.
The mechanism of dielectric breakdown of foil-wound type oil-filled capacitors is generally considered as follows:
Oil-filled capacitors are made by properly selecting the combination of an insulating oil and a solid insulating material such as film or paper and the impregnation is carefully carried out to avoid the contamination with water and foreign materials and the formation of voids such as un-impregnated portions or bubbles. In such an oil-filled capacitor, the partial discharge is caused to occur locally, wherein gases, mainly hydrogen gas, are generated and they are diffused or absorbed in the peripheral regions, otherwise, the partial discharge will increase and finally the dielectric breakdown occurs. The portions to initiate the discharge are mainly in the peripheral ends of electrode foils. The concentration of electric field is caused to occur in the portions in which adjoining electrode foils are irregularly arranged by several tens microns or in the projections in micron order in the cut end portions of electrode foils. When these portions are insufficiently covered by an insulating oil, the partial discharge occurs. The portion suffered by the partial discharge sometimes spreads from one point, or in some case, the partial discharge occurs in many portions simultaneously.
Meanwhile, the separating out of crystals from a liquid insulating oil is also initiated irregularly. In many cases, the crystallizing out begins in a manner to deposit crystals on foreign substances other than the insulating oil such as solid insulating materials, electrode foils, and solid particles suspended in the liquid phase. When crystals are once formed, they play seeds for succeeding separating out of crystals, so that the solid phase (crystalline phase) in the liquid is increased. It is considered that the solid phase like this exists locally and irregularly in the liquid insulating oil.
The relation between the existence of solid phase and the local discharge will be discussed. Assuming that the quantity of the solid phase and the occurrence of local discharges are the matters of probability, even in a system in which the solid phase scarcely exists or produced, it cannot be avoided in view of probability that the solid phase sometimes exists in a portion where the concentration of electric field occurs, or that the insulation becomes insufficient with inviting the local discharge. In other words, the existence of any amount of solid phase (crystals) in a liquid cannot be allowed in order to avoid the local discharge.
In view of the above, if an insulating oil composition in which no crystallization occurs at all at low temperatures is prepared from the bicyclic aromatic hydrocarbons having 14 carbon atoms as shown in Table 2, though it cannot be said to be impossible but may be said to be impractical because the ranges of selection of the composition are quite narrow.